This is a little trick to do while making ice cubes that will make them make a momentary crackling sound when dropped in water.
First, add water to an ice cube tray, and place it in the freezer as normal. However, after twenty minutes, take out the tray and bend it, as if you were trying to remove full grown ice cubes. At this point, the ice cubes should be frozen over the top, but still have liquid water in the center Then put it back in the freezer.
For whatever reason, (I think perhaps that this creates a little air pocket right near the surface that escapes when the ice cube is immersed in water) this produces an ice cube, that, when put in water, cracks. It is surprisingly loud. Makes a great practical joke. | 
